1. Field of the Invention
The present invention relates to a method of producing a tubular thermoplastic resin composite bead, a tubular expandable thermoplastic resin composite bead, a tubular expanded thermoplastic resin composite bead, and a foamed molded article formed from expanded thermoplastic resin composite beads with continuous bore spaces.
2. Description of the Related Art
Foamed molded articles, which are obtained by mutually fusing and bonding thermoplastic resin expanded beads in a mold for shaping, are widely used in various applications such as packaging materials, construction materials, and impact energy absorbing materials for vehicles because of their excellent properties like energy absorbing properties, light-weight properties, thermal insulating properties, etc.
There is proposed, as the foamed molded article, a styrene modified olefin resin foamed molded article that employs a styrene modified olefin resin as a base resin (see Patent Documents 1-6). The styrene modified olefin resin foamed molded article is obtained by using expandable resin beads that are prepared, for instance, in a way described below.
That is, olefin resin beads composed of polyethylene, for example, are impregnated with vinyl aromatic monomers such as styrene monomers. Then, the vinyl aromatic monomers are polymerized in the olefin resin beads, thereby obtaining a styrene modified olefin resin beads.
Thereafter, the styrene modified olefin resin beads are impregnated with a hydrocarbon blowing agent such as propane, butane, and pentane. As a result, the expandable resin beads that employ a styrene modified olefin resin as a base resin are formed.
Where butane is used as a blowing agent for the styrene modified olefin resin beads, the speed with which butane is dissipated from the base resin is relatively high, so the conventional preexpansion is usually performed at the same factory that expandable resin beads are manufactured to produce the expanded beads, and then the expanded resin beads are conveyed to factories where a molded article is produced. Instead, where pentane is used as a blowing agent, the speed with which pentane is dissipated from the base resin is slower than the case where the butane is used, so the expandable resin beads may be conveyed without preexpansion to produce expanded beads for molding is performed at a factory where a molded article is produced. In either case, a foamed molded article formed from expanded resin beads can be produced by in-mold molding.
Accordingly, in the case where a styrene modified olefin resin is employed as a base resin to produce expandable resin beads prepared to manufacture a foamed molded article, equipment for producing a molded article formed with expanded styrene resin beads, for example, is partly or wholly utilized. Because of this, initial investment is not required for producing a foamed molded article formed from expanded polyolefin resin beads, and a foamed molded article formed from expanded beads may be produced in a way to have improved chemical resistance and toughness by using expanded styrene modified olefin resin beads. The foamed molded article formed from expanded beads has been widely used in various applications such as impact energy absorbing materials such as bumpers and tibia pads, cushioning materials for home electronic appliances, and containers for transporting liquid crystal glass panels.
There is a type of foamed molded article formed from expanded beads that has a high porosity. A general foamed molded article (i.e., a foamed molded article in which bore spaces are not formed) is formed by performing post-expanding of expanded beads, so that the expanded beads are fusion bonded together in a way to fill voids formed between the expanded beads. In contrast, a foamed molded article with high porosity has been formed by fusion-bonding the expanded beads so as not to allow the voids formed between the expanded particles to be filled thoroughly. In order to obtain such a high porosity of foamed molded article, special molding equipment and control were required in the past. Due to this, it was difficult to produce a foamed molded article having a high porosity, and excellent mutual fusion properties among the expanded beads according to the conventional production methods of in-mold molding.
In view of this, there is proposed a method of producing a foam molded article having continuous bore spaces as described above without using special equipment. According to this method, a plurality of foam chips, having a length of 2 cm or more in its longest portion and a non-spherical shape, are mutually fused together in a mold (see Patent Document 7).
Further, there is proposed a method of molding tubular expanded beads in a mold. According to this method, first, polyolefin resin is melted by an extruder. Then polyolefin resin is extruded in a tubular strand-shape from a die attached to a tip end of the extruder and cut. Thereby tubular resin beads are obtained. Thereafter, the tubular resin beads are dispersed in an aqueous medium in a closed pressure vessel, such as an autoclave, and a physical blowing agent is pressed into the pressure vessel. After that the softened tubular resin beads containing the blowing agent are released from the pressure vessel together with the aqueous medium to obtain the tubular expanded beads. And the tubular expanded beads are molded in a mold (see Patent Document 8).    Patent document 1: JP49-97884 Unexamined Patent Publication (Kokai)    Patent document 2: JP52-32990 Unexamined Patent Publication (Kokai)    Patent document 3: JP45-32623 Examined Patent Publication (Kokoku)    Patent document 4: JP01-284536 Unexamined Patent Publication (Kokai)    Patent document 5: JP48-101457 Unexamined Patent Publication (Kokai)    Patent document 6: JP49-5473 Unexamined Patent Publication (Kokai)    Patent document 7: JP05-177723 Unexamined Patent Publication (Kokai)    Patent document 8: JP08-108441 Unexamined Patent Publication (Kokai)